1. Field of the Invention
The invention relates to corrosion resistant zirconium alloys for extended burnups in water-moderated nuclear reactor core structural components and fuel cladding but which also may find utility in zirconium alloys for other analogous corrosive environment uses.
2. Description of the Prior Art
In my previous invention of a DUCTILE IRRADIATED ZIRCONIUM ALLOY, U.S. Pat. No. 4,879,093, an improved ductile zirconium-tin (Zircaloy) --niobium or molybdenum alloy for use in water moderated nuclear reactor core structural components and fuel cladding is disclosed. Because of that invention, longer in-reactor residence times and extended fuel burnup--above 55 GWD/MTU--were made practical because the alloy's stabilized microstructure minimized loss of alloy ductility. The improved ductility is required to resist release of fission gases and to handle spent fuel safely. That alloy retained reasonable corrosion resistance because of its primarily alpha phase structure and optimum intermetallic precipitate average particle size. Improvement of in-reactor corrosion resistance of zirconium alloys at extended burnups is still an area of technology with "room for improvement", however.
There are several possible approaches or objectives to enhance corrosion resistance at extended burnups for structural components and fuel cladding of water-moderated nuclear reactors, (a term used generically for PWR, BWR and PHWR). They are generally listed as follows:
1. Decrease the hydrogen adsorbed at the water-oxide interface;
2. For a given amount of hydrogen adsorbed in the oxide, slow its diffusion toward the metal substrate;
3. Create higher hydrogen solubility in the metal substrate to reduce precipitation;
4. Reduce the percentage fraction of hydrogen which is absorbed by the metal substrate; and,
5. Decrease the basic oxidation rate of the metal substrate to avoid degradation of the metal substrate's corrosion resistance.
An understanding of these approaches or objectives, and a recognition that in-reactor corrosion resistance of zirconium alloys at extended burnups is degraded due to the fracture of hydride precipitates at the metal-oxide interface, has led to the present invention of a corrosion resistant zirconium alloy for use in water moderated nuclear reactors.